1. Field of the Invention
The present invention relates to an image shifting apparatus for reducing the vibration of an object image by shifting the object image on the photographing plane of a photographing optical system.
2. Related Background Art
In a conventional photographing device such as a camera, even a slight movement of a photographer during a photographing operation (exposure) causes an object image on a photographing plane (e.g., a film surface) to shift. The resultant photograph is a so-called blurred photograph with unclear contours. In order to solve such a problem, an image shifting apparatus has recently been studied and developed. This apparatus reduces the vibration of an object image on a photographing plane (this operation will be referred to as "vibration reduction" hereinafter) by shifting a vibration reduction optical system constituted by part or all of a photographing optical system during an exposure operation so as to counteract the movement of the photographer.
As such an image shifting apparatus, for example, Japanese Laid-Open Patent Application No. 3-110530 (the counterpart of which is U.S. Pat. No. 5,172,276) discloses an apparatus for performing vibration reduction by shifting a vibration reduction optical system (to be referred to as a "VR lens" hereinafter) within a plane perpendicular to the optical axis of a photographing optical system. In this apparatus, the VR lens is supported within a plane perpendicular to the optical axis by an X-Y stage movable in the X and Y directions perpendicular to the optical axis, and two motors are used as drive sources for driving the VR lens. The rotational motions of the motors are slowed down by predetermined reduction gears and converted into two linear motions which are not parallel to each other by lead screws or the like. The resultant motions are transferred to the X-Y stage. As a result, the VR lens is moved within a plane almost perpendicular to the optical axis.
When the X-Y stage is used in the above manner, the VR lens may not be smoothly moved because of friction produced on the X-Y stage. In order to solve such a problem and ensure smooth movement of the VR lens, a method of supporting the VR lens in a lens barrel with elastic members has recently been used. More specifically, a plurality of cylindrical elastic members are arranged in the lens barrel such that the axis of each cylindrical member becomes parallel to the optical axis, and the VR lens is cantilevered on end portions of the elastic members. With this structure, the VR lens can be smoothly moved within a plane defined by the end portions of the elastic members.
When a rotary motor such as a DC motor is used as a drive source, the rotational motion of the motor must be slowed down and converted into a linear motion by using gears. With these gears, however, it is difficult to prevent generation of noise. In contrast to this, there have recently been strong demands for photographing devices, especially home photographing devices capable of low-noise operations. Under the circumstances, the use of a voice coil motor (to be referred to a "VCM" hereinafter) as a drive source for a VR lens has been proposed. The voice coil motor can directly supply a linear drive force to the VR lens without the mediacy of any mechanical mechanism. The VCM is a motor for generating a drive force by using an electromagnetic force generated by a current and a magnetic force. This motor is characterized in that it generates a linear drive force proportional to the magnitude of a current for energization.
In the above conventional image shifting apparatus, since the VR lens is supported by the elastic members, if, for example, the photographing device is held such that the axis of each cylindrical elastic member is set horizontally, the VR lens moves downward owing to the effect of the weight of the lens. As a result, the center of the VR lens is offset from the optical axis in a natural state. For this reason, in the conventional image shifting apparatus, even if no vibration reduction is to be performed, the VCMs must be driven to generate a force against the gravity in order to make the center of the VR lens coincide with the optical axis. As a result, the conventional image shifting apparatus greatly consumes power. Therefore, for example, in a portable photographing device using only a built-in battery having a limited capacity, the service life of the battery is shortened.
In order to solve the above problem, there has been proposed a method of setting the start position of vibration reduction driving (to be referred to as "VR driving" hereinafter) at the position where the elastic force of the elastic members for supporting the VR lens balances with the gravity acting on the VR lens and the like. When VR driving is performed by this proposed method, since the initial position of the VR lens is offset downward from the optical axis, the VR lens can move to a position separated further from the optical axis in the gravitation direction. For this reason, when the VR lens is driven in the gravitation direction, the resultant image tends to deteriorate because of an aberration. In addition, if driving of the VR lens is stopped within a range in which no aberration is caused, the range of vibration which can be reduced is narrowed.
When the VR lens is moved in the above manner, although the object image can be moved within the photographing plane, the image on the photographing plane is influenced by an aberration with an increase in the moving amount of the VR lens. As a result, the object image deteriorates. The degree of the influence of an aberration varies depending on the performance of an optical system to be used. In any case, the VR lens cannot be moved unlimitedly, and must be moved within a predetermined range.
In conventional image shifting apparatuses, therefore, various methods are used to limit the moving ranges of VR lenses. For example, a conventional method of mechanically limiting the moving range of a VR lens (this operation will be simply referred to as "correction limitation" hereinafter) is known. This method uses a rotary motor as a drive source for the VR lens. The rotational motion of this motor is slowed down by reduction gears and converted into a linear motion, which is used as a drive force for the VR lens. Limiting members are arranged around the VR lens at predetermined intervals. When the VR lens is moved by the above drive force by a predetermined distance or more from the optical axis, the movement is inhibited by the limiting members, thus performing correction limitation.
Another method is disclosed in Japanese Laid-Open Patent Application No. 3-110530. According to Japanese Laid-Open Patent Application No. 3-110530, an image shifting apparatus includes a disk integrally mounted with a portion of a reduction gear train engaged with the pinion gear of a pulse-driven rotary motor as a drive force for a VR lens. This disk has one hole. This image shifting apparatus also includes a photointerrupter mounted to sandwich a peripheral portion of the disk with the hole. When the image shifting apparatus drives the VR lens, i.e., the rotary motor produces a rotational motion, the photointerrupter detects the rotational position of the disk on the basis of light beam outputs transmitted through the hole of the disk. In this case, the detected outputs are used to detect the rotational position of the pinion gear of the motor. With this operation, the motor is controlled such that the number of drive pulses representing the rotational position of the motor and output from the photointerrupter does not exceed a predetermined value. In this manner, correction limitation is performed.
In the above image shifting apparatus for mechanically performing correction limitation, correction limitation is not performed by stopping the motor, but is simply performed by mechanically inhibiting the movement of the VR lens using the limiting members. For this reason, power for driving the VR lens is wasted. This poses a serious problem in a portable photographing device such as a camera incorporating a battery having a limited capacity as a power source.
In the image shifting apparatus for performing correction limitation on the basis of the number of pulses detected by the photointerrupter, a complicated control system must be used for correction limitation. It is therefore difficult to propose an inexpensive image shifting apparatus.